The most common technique for error detection of non-real time services is based on Automatic Repeat request (ARQ) schemes which is combined with Forward Error Correction (FEC), called hybrid ARQ. If an error is detected by Cyclic Redundancy Check (CRC), the receiver requests the transmitter to send additional bits. From different existing schemes the stop-and-wait (SAW) and selective-repeat (SR) continuous ARQ are most often used in mobile communication.
A data unit (PDU) will be encoded before transmission. Depending on the bits that are retransmitted three different types of ARQ are e.g. defined in S. Kallel, R. Link, S. Bakhtiyari, IEEE Transactions on Vehicular Technology, Vol. 48 #3, May 1999 “Throughput Performance of Memory ARQ Schemes”.                Type I: The erroneous PDU is discarded and a new copy of that PDU is retransmitted and decoded separately. There is no combining of earlier and later versions of that PDU.        Type II: The erroneous PDU that needs to be retransmitted is not discarded, but is combined with some incremental redundancy bits provided by the transmitter for subsequent decoding. Retransmitted PDU's sometimes have higher coding rates and are combined at the receiver with the stored values. That means that only little redundancy is added in each retransmission.        Type III: Is the same as Type II only that every retransmitted PDU is now self-decodable. This implies that the PDU is decodable without the combination with previous PDU's. This is useful if some PDU's are so heavily damaged that almost no information is reusable.        